Recently, with increasing need for small high output light emitting devices, demand for large flip-chip type or vertical type light emitting devices securing good heat dissipation efficiency has also be increased. In the flip-chip type or vertical type light emitting device, electrodes are directly joined to a secondary substrate, thereby providing better heat dissipation efficiency than a lateral type light emitting device. Thus, the flip-chip type or vertical type light emitting device can effectively transfer heat to the secondary substrate upon application of high current and thus can be suitably used for a high output light source.
In a typical flip-chip type or vertical type light emitting device, since a main light emitting plane is a surface of an n-type semiconductor layer on which a growth substrate resides, a structure capable of reflecting light towards the light emitting plane is formed on a p-type semiconductor layer. For this reason, an Ag electrode capable of forming ohmic contact with the p-type semiconductor layer while acting as a reflector is generally disposed on the p-type semiconductor layer.
However, the structure wherein such a reflective electrode is used as the p-type electrode has the following problems.
First, in formation of the reflective electrode, a metal layer is subjected to patterning through photolithography, which makes it difficult to cover the overall surface of the p-type semiconductor layer with Ag due to process margin thereof. Namely, an ohmic contact area between the p-type semiconductor layer and an Ag electrode is decreased. As a result, a current injection area of the light emitting device is decreased and light is not reflected in a region in which the reflective metal is not formed, thereby deteriorating luminous efficacy of the light emitting device.
Moreover, Ag atoms of the reflective electrode contacting the p-type semiconductor layer diffuse into the p-type semiconductor layer and act as impurities. Particularly, such Ag atoms can diffuse into the p-type semiconductor layer through defects (for example, dislocations) of the p-type semiconductor layer. The diffused atoms deteriorate crystallinity of the semiconductor layer while increasing a possibility of current leakage, thereby deteriorating electrical and other characteristics of the light emitting device.
Therefore, there is a need for a novel light emitting device exhibiting excellent electrical and optical characteristics.